(1) Field of the Invention
The present invention relates to devices for modulating waves. In particular, the present invention relates to devices that use a pair of complementary zone plates to perform such modulation.
(2) Brief Description of Prior Art
In recent years, spot array generators have attracted lots of attentions in the fields of high-speed and high-resolution image generation owing to their unique capabilities of being able to generate large field images with a very high resolution by providing a massive amount of image pixels in a parallel fashion.
Shown in FIG. 1 is an illustration of a conventional spot array generating system 100 having 4 pixels. A spot array 102 is an image formed by a lens array 104 on the array's focal plane 106 when the lens array 104 is illuminated by light beams 108. The lenses in the array can be either refractive or diffractive. In order to generate dynamic images for the purpose of high-resolution image generation, it is necessary to provide means for modulating the individual light spots in the spot array. In the conventional spot array generating systems as shown in FIG. 1, a spatial light modulator (SLM) 110 is placed in series with the lens array 104 and is used to modulate the light intensity of each spots. Additional components such as lens systems 112, and aperture arrays 114 may be needed in order to provide the pixel-to-pixel matching between the SLM 110 and the lens array 104 and to eliminate unwanted light.
For example, Darioi Gil, et al., J. Vac. Sci. Technol. B(20), p 2597(2002), demonstrated a lithographic apparatus employing a combination of a Grating Light Valve array and a zone plate array. Kenneth C. Johnson (U.S. Pat. No. 6,177,980, issued on Jan. 23, 2003) proposed a lithographic system using a combination of various SLM arrays with a microlens array. Even though the functionality of some of these combinations has been successfully demonstrated, there are serious limitations to these approaches. Commercial applications usually require a resolution that is at least as high as the resolutions of today's commonly used display formats. For example, it is preferable to use a display having a resolution of 1024×768 pixels, or a total of 786,432 pixels. To combine a lens array with a SLM, both of which have a very large number of pixels, precise alignment between the components is critical. Such a mechanical arrangement is vulnerable to environmental effects such as temperature changes and mechanical vibrations. Therefore, the precise alignment is often difficult to maintain. Besides, the needs for additional necessary optical components such as lens systems and aperture arrays result in the loss of optical efficiency, increased system size, complexity and cost. The invention of Henry Smith (U.S. Pat. No. 5,900,637, issued on May 4, 1999) addressed some of the issues by integrating a micro-shutter array with a zone plate array. The proposed micro-shutters electronically rotate a micro-plate substantially in or out of the light path of each lens elements in order to turn the corresponding light spot OFF or ON. This type of SLM is not expected to have a fast response time due to the required large mechanical motion of the micro-plate in order to achieve a full modulation. And the fabrication process of the integrated lens array and micro-shutters is quite complex.
Spot array generating systems using a combination of a lens array and a SLM have many other disadvantages. For example, microlens arrays are arrays of diffractive lenses. They are difficult to manufacture, especially for large arrays of diffraction limited lenses. Their fabrication process is not compatible at all with the CMOS technology. On the other hand, existing SLMs have their shortcomings as well. Digital micromirror arrays, such as the DLP™ manufactured by Texas Instruments, TX, is not very fast due to the large motions needed to modulate the light beams. The Grating Light Valve (GLV) array manufactured by Silicon Light Machines, CA, has a very short response time. However due to the asymmetrical ribbon design, GLVs are not appropriate for 2D arrays and are also not polarization independent. Liquid crystal based SLMs modulate light by changing the orientation of molecules, and consequently have significant polarization losses.
Therefore, for applications of high performance image generation systems, it is desirable to have a spot array generating device that is very compact, reliable, and easy to use. Preferably it is a single integrated device that is capable of both modulating and focusing an incoming wave. It is also desirable to have a spot array generating device that is suitable for 2-D arrays, has a very short response time, and is polarization independent. It is still desirable to have a spot array generating device that is low cost, easy to fabricate, and compatible with CMOS technology. Currently there is no prior art that satisfies these features numerated above.
Fresnel zone plates, or zone plates for short, were invented more than a century ago. As described by most optics textbooks, a zone plate is a transparent plate with a set of concentric zones made opaque to the incident wave. This type of zone plates is sometimes referred to as amplitude zone plates. The outer radius Rn of the nth zone is determined by the following zone plate equation
      R    n    =            (                        n          ⁢                                          ⁢          λ          ⁢                                          ⁢          F                +                                            n              2                        ⁢                          λ              2                                4                    )              1      /      2      where n is a positive integer, λ is the wavelength of the incident wave, and F is the focal length of the primary focal point of the zone plate. It commonly refers to the zones that have an even index n as the even zones, and the zones that have an odd index n as the odd zones. The size ds of the diffraction limited spot formed by a zone plate is governed by the same equation for refractive lenses, ds=λ/NA, where NA is the numerical aperture of the zone plate. A zone plate is a diffractive optics. It also behaves like a lens capable of forming images and focusing waves.
Illustrated in FIG. 2 is a typical transmissive amplitude zone plate 140 under the illumination of an incident wave. The incident wave 142 is diffracted by the clear zones 144 into a series of focal points 146, 147, and 148, with most of the energy being at the first or the primary focal point 148. A corresponding set of virtual focal points, formed on the right-hand side of the zone plate, is not shown here for clarity. Such a zone plate also carries a substantial amount (˜25%) of plane wave component in the forward direction. Since half of the incident wave is blocked by the opaque zones 149, this type of zone plates is not very efficient. To overcome the problem of low efficiency, a phase-reversal zone plate was proposed in 1888 by Lord Rayleigh and demonstrated in 1898 by R. W. Wood, Philos. Mag. V45, 51(1898). The basic idea of a phase-reversal zone plate is to convert the opaque zones into transparent zones, and also change the phase of the waves passing through them by 180 degrees. A constructive interference between the diffracted waves coming from the clear zones and those coming from the phase-reversed zones results in a four-fold increase in the intensity of waves at the focal points. Phase-reversal zone plates are often simply referred to as phase zone plates. Phase zone plates have been used frequently in X-ray optics.
It should be noted that all of above descriptions for transmissive zone plates can be used to describe the properties of reflective zone plates. For example, the waves leaving a reflective zone plate consist of both diffracted and reflected waves. The diffracted waves form a series of focal points along the axis of the reflective zone plate.
A major limitation of both amplitude zone plates and phase zone plates is that the intensities at the focal points of the zone plates are fixed once the zone plates are made, and can not be changed dynamically. Improvements by several inventors provided certain degree of dynamics to zone plates. For example, Dennis S. Greywall (U.S. Pat. No. 5,684,631, issued on Nov. 4, 1997) invented an optical modulator/switch including a reflective zone plate. The modulator modulates a light beam by changing the orientation of the reflective zone plate, therefore the angular positions of the focal points of the zone plate. This type of modulator/switch can be used for multiplexing or switching optical signals. However, it is not suitable for generating spot arrays. George W. Webb (U.S. Pat. No. 5,360,973, issued on Nov. 1, 1994) proposed a dynamic zone plate deflector made of a semiconductor material for forming and scanning millimeter wave radiation. The idea is to spatially modulate the density of charge carriers, and therefore the dielectric property of the semiconductor material. However, the invention can not be used for radiation of shorter wavelength such as visible light or ultraviolet light since these radiation can not pass through the semiconductor materials. Toshio Naiki (U.S. Pub. No. 2002/0071172, published on Jun. 13, 2002) disclosed a diffractive optical device based on the acoustooptical effect of certain materials such as Lithium Niobate. By changing the frequency of a radio frequency (RF) signal applied to a circular interdigital transducer fabricated on an acoustooptical material, the focal length of the diffractive optical device is changed or modulated. Since the acoustooptical effect is a weak effect (only a few percent), the modulation is quite small. The requirement of RF electronics also makes it impractical to integrate such diffractive optical devices into a large array. Therefore, prior art of dynamic zone plates failed to provide a modulator that is suitable for being used as spot array generators.